###Live Caption:A Long-billed Curlew glides above the water in the Tijuana Estuary in a Tuesday Nov. 27, 2007 photo, in Imperial Beach, Calif. The Curlew and other wildlife habitants of the estuary are threaten by the increasing pollution from Tijuana, Mexico. (AP Photo/Lenny Ignelzi)###Caption History:A Long-billed Curlew glides above the water in the Tijuana Estuary in a Tuesday Nov. 27, 2007 photo, in Imperial Beach, Calif. The Curlew and other wildlife habitants of the estuary are threaten by the increasing pollution from Tijuana, Mexico. (AP Photo/Lenny Ignelzi)###Notes:###Special Instructions:

Photo: Lenny Ignelzi

###Live Caption:A Long-billed Curlew glides above the water in the...

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###Live Caption:Pollution that has washed down stream from Tijuana, Mexico and eventually run into the Tijuana Estuary is seen in a Sunday Dec. 23, 2007 photo in Imperial Beach, Calif. The estuary faces critical levels of pollution that threaten wildlife and their habitat. (AP Photo/Lenny Ignelzi)###Caption History:Pollution that has washed down stream from Tijuana, Mexico and eventually run into the Tijuana Estuary is seen in a Sunday Dec. 23, 2007 photo in Imperial Beach, Calif. The estuary faces critical levels of pollution that threaten wildlife and their habitat. (AP Photo/Lenny Ignelzi)###Notes:###Special Instructions:SUNDAY DEC. 23, 2007 PHOTO

Photo: Lenny Ignelzi

###Live Caption:Pollution that has washed down stream from Tijuana,...

Along the U.S.-Mexico border, the fragile salt marshes at the mouth of the Tijuana River are clinging to life as one of the last vestiges of undeveloped California coast, where tall grasses sway gently in the breeze and rare birds stop to nest.

Just across the border is another landscape: old tires, plastic bottles, raw sewage and vast amounts of filthy sediment, all of it threatening to wash across the divide and spoil one of California's few surviving coastal wetlands.

Conservationists are struggling to protect the teeming nature preserve south of San Diego from the loose soil, trash and pollutants that have sometimes spilled into the river from shantytowns upstream in rapidly growing Tijuana.

With more than 90 percent of California's wetlands already lost to urban encroachment, "we're desperate to save every little scrap of coastal salt marsh we can," said Jeff Crooks, a research coordinator at the preserve.

At 2,500 acres, the Tijuana Estuary is three times the size of New York's Central Park, and for years it has remained largely undisturbed despite being encircled by development on both sides of the border.

This year, flash floods have washed silt and garbage from Tijuana into three catch basins on the U.S. side built to trap debris before it seeps into the federally protected preserve.

The basins can store 60,000 cubic yards of debris - enough to fill 6,000 dump trucks. But the pits are nearly full, and any more rain could cause the debris to overflow into the estuary.

The estuary, largely managed by the California and U.S. governments, was designated a "wetland of international importance" in 2005.

The pollution problem has grown more urgent in recent years with the rapid growth of Tijuana, a city of at least 1.3 million people that grows an average of 80,000 new residents each year.

Romo leads a cross-border effort to clean up a section of Tijuana's Los Laureles Canyon, a dirt-road community of 800 shanties built on steep slopes using scrap metal and used tires. When it rains, sediment and other debris flow across the border into the catch basins.

The sediment poses the most serious threat. During the winter of 2004-05, silt and sand from the canyon burst through the basins and buried 18 acres of salt marsh.

"The sediment problem is something we battle with every rainstorm," estuary manager Clay Phillips said. "I used to love the rain, but now I cringe and think, 'Are we going to lose marshland?' "

Romo's work is one of several conservation projects at the estuary. Other efforts are focused on restoring 250 acres of sediment-filled marshland and removing the invasive tamarisk shrub that competes with native plants.

Last month, Romo and a team of students from UC San Diego, where he teaches urban planning, helped residents build roads that trap water in the ground instead of creating runoff that ends up in the estuary. It's a slow process. Only about a third of the 100,000 concrete blocks needed for the new pavement have been installed so far.

Many Tijuana residents were skeptical about the cross-border cleanup, which is funded by the U.S. and Mexican governments and a private foundation. To ease distrust, Romo holds workshops to teach residents about their environmental footprint.

Some scientists say the cleanup addresses only a small part of the problem because most of the polluted water infiltrating the preserve comes from the area's main watershed, three-quarters of which is in Mexico.

But if it is not protected, experts worry about the potential loss of plant and wildlife diversity. Sand dollars and steelhead trout that were once abundant in the estuary have now all but disappeared.

Joy Zedler, a botanist at the University of Wisconsin who has done research in the preserve since the 1970s, found last year that exotic plants were crowding out native ones, changing the ecosystem and altering the food chain.